1. Field of the Invention
The present invention relates to dual chamber cardiac pacemakers having multiple single chamber and dual chamber pacing modes, and more particularly to a method and apparatus for effecting mode switching between selected primary pacing modes.
2. Description of the Background Art
Dual chamber pacing modes have been widely adopted for pacing therapy in both external, bedside pacing systems and implantable pacing systems. Among the dual chamber pacing modes is the "DDD" mode, which can pace the atrium and ventricle, senses both the atrial and ventricular depolarization signals (P-waves and R-waves, respectively), and can either inhibit or trigger pacing stimuli for both chambers. Other dual chamber pacing modes include the DVI pacing mode which can pace the atrium and ventricle, but senses only R-waves, and can either inhibit or trigger ventricular pacing stimuli, and the VDD pacing mode which can pace the ventricle, senses both P-waves and R-waves, and can inhibit ventricular pacing stimuli. These three letter mode codes are standardized in the pacing industry as "NBG" codes published in 1987 by the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG). The first letter denotes the heart chamber paced, the second letter denotes the chamber sensed and the third letter denotes the function in response to a sensed event. The letters A, V, D, T and I are employed wherein "A" specifies an atrial function, "V" specifies a ventricular function, "D" specifies both A and V functions, "T" means a triggered operation, and "I" means an inhibited operation. The three letter code is expanded by a further letter "R" to signify a mode of operation involving establishing pacing rate in response to a physiologic sensor representing a patient's level of activity or need for cardiac output.
The three letter code is often used (1) to generally characterize a type of pacemaker, (2) to signify a programmed primary pacing mode of a multi-mode, implantable programmable, pacemaker or the manually set, primary pacing mode of a multi-mode, external pacemaker, or (3) to indicate the current pacing mode within the family of possible current pacing modes included inherently within a programmed or set primary pacing mode that largely depends on the behavior of the patient's heart in relation to the adjustable pacing, sensing and rate parameters of the pacemaker pulse generator as described below. In the following description of the background art and the invention, unless otherwise noted, references to the pacing mode are to the primary pacing mode within the family of possible primary pacing modes of what is generally characterized as a DDD dual chamber pacemaker type not having a rate response capability. Moreover, references to "mode switching" made herein are to be distinguished from the term as used in describing the modes of operation attendant to establishing a pacing rate in response to a physiologic sensor rather than in response to an atrial depolarization or a programmed lower pacing rate.
A DDD dual chamber pacemaker in the DDD pacing mode requires a pulse generator having atrial and ventricular pace/sense leads attached thereto. The pulse generator in the DDD pacing mode requires an atrial sense amplifier to detect P-waves conveyed through the atrial pace/sense lead, a ventricular sense amplifier to detect R-waves conveyed through the ventricular pace/sense lead, a timing and control system for establishing pacing modes and time intervals for sensing and pacing operations, and ventricular and atrial output circuits to provide atrial and ventricular pacing pulses to the appropriate lead. If the P-wave is not sensed within a predefined time interval (atrial escape interval), the pacemaker supplies an atrial pacing pulse to the atrium through the atrial pace/sense lead. The atrial escape interval may be characterized in terms of a ventricular-to-atrial (V-A) interval. Following an atrial event (either sensed or paced), the timing and control system establishes an atrial-ventricular (A-V) interval. If the A-V interval lapses before an R-wave is sensed, the pulse generator supplies a ventricular pacing pulse to the ventricle through the ventricular pace/sense lead. The combined V-A and A-V interval that the pacemaker is set to operate at is sometimes referred to as the A--A interval or the V--V interval or the lower rate interval.
Pacemakers operating in the DDD pacing mode have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates between the lower rate and an upper rate limit. For this reason, the DDD mode is typically the primary pacing mode of a dual chamber pacing system that is programmed into an implantable pulse generator or manually set for an external pacemaker. The related DVI or VDD modes may be programmed or manually set to be the primary pacing mode if the patient has little or no underlying atrial sinus rhythm or intermittent A-V conduction with intact sinus rhythm.
Such DDD pacing systems are also typically programmable or operable in single chamber pacing modes, typically the AAI and VVI modes, where sensing and pacing are restricted to the atrium and ventricle, respectively. In practice, a very small percentage of implanted DDD pulse generators are programmed into these single chamber modes unless a problem arises with sensing or pacing through the atrial or ventricular lead bearing electrodes.
In external pacemakers typically used during a patient stay in a medical facility, the DDD pacing mode architecture provides the economy and flexibility of being used for either the single chamber or the dual chamber modes. The appropriate temporary lead system may be positioned in the patient, depending on the patient need, and the pulse generator may be switched to operate in one of the modes prescribed by the attending physician. For example, during open heart surgery, removable, temporary atrial and ventricular electrode bearing leads may be attached to the heart for post-operative DDD pacing during the recovery period and until the leads are removed. If one of the electrodes is dislodged from the heart, it may be necessary to switch to the appropriate single chamber pacing mode for the remaining electrode. In other cases, temporary single chamber pacing may be prescribed from the outset. Such typical external pacemaker pulse generators include the Medtronic.RTM. Model Nos. 5342, 5345 and 5346 external pacemakers, the Telectronics.RTM. Model 4553 external pacemaker, and the Pacesetter Systems, Inc. Model 3070 external pacemaker.
The pacing mode is typically selected in these external pulse generators through the adjustment of a mode setting switch. When the mode is selected, the physician also sets appropriate pacing parameters for the pacing mode. The pacing parameters set include atrial and/or ventricular sense amplifier sensitivities to the P-wave and/or R-wave signal to provide a suitable amplification to disregard background noise and reliably provide true A-sense and/or V-sense trigger signals to the timing and control system. The pacing parameters set also include atrial and/or ventricular pulse generator output pulse amplitudes to provide a suitable safety margin over the capture threshold for the atrium and/or ventricle.
If it becomes necessary to adjust the pacing mode or the parameters due to a change in the patient's condition or dislodgement of a temporary lead, the changes may have to be made by attending staff that are not as familiar with the pacing modes or setting of the sensitivity and pacing amplitude parameters. In addition, in any case, some medical personnel do not understand the NBG code and find the pacing modes difficult to understand. Confusion may occur and the wrong settings may be made. A need exists for a simplified manner of setting the pacing modes and parameters of external DDD pulse generators.